X-ray mammography is generally regarded as the single most important tool in the early detection of breast cancer. Mammography detects 85-90% of breast cancers, and the American Cancer Society recommends that women age forty and older undergo yearly screening. The main drawback of mammography is that it has a low positive predictive value which frequently necessitates additional testing, such as more intensive mammography, magnetic resonance (MR) imaging, ultrasound and/or biopsy.
MR imaging produces higher resolution images of deeper and/or denser tissue than mammography, without the use of ionizing radiation. However, MR instruments are expensive to own and operate, and the resulting images suffer from the same low positive predictive value as mammograms. Masses detected by MR imaging require further evaluation, such as by ultrasound, which can distinguish between solid tumors and cysts, and/or biopsy, which is an invasive and unpleasant procedure.
As an alternative to biopsy, optical imaging of tissue is an emerging modality for detection, diagnosis and monitoring of breast cancer. Diffuse optical tomography uses electromagnetic energy, ranging from visible light to near infrared (NIR), to probe objects beneath the skin surface, such as tissue, fluid and tumors. Information about tissue composition and morphology is gained by measuring and modeling light absorption, scattering and emission. This information can be used to create two-dimensional cross-sectional slices and/or three-dimensional images, and also to distinguish between cancerous tumors, non-cancerous tumors, calcifications and cysts. For example, when light is absorbed by a compound (chromophore) within the tissue, the chromophore, such as hemoglobin, lipid or water, can be identified and quantified. Tumors frequently have increased blood flow, so that a high concentration of hemoglobin may be indicative of a tumor. Further, intense scattering of light may be attributed to a solid or semi-solid mass, such as a tumor.
Qualitative, moderate-resolution optical images have been used to diagnose tumors based on their metabolic and functional status, but improvements in quantitative accuracy and resolution may be obtained when anatomical information from other modalities, such as MR imaging or ultrasound, is used in the image reconstruction procedure. Hybrid instrumentation and methodologies for incorporating anatomical information as spatial priors into tissue reconstruction algorithms are currently being developed, and can significantly improve the accuracy of the recovered information by identifying borders between different tissue types, as observed by MR.
One hybrid system for combined optical and MR imaging is described in V. Ntziachristos, X. H. Ma, and B. Chance, “Time-correlated single photon counting imager for simultaneous magnetic resonance and near-infrared mammography”, Rev. Sci. Instrum., 69(12), 4221-4233, December 1998. This system utilizes a pair of soft compression plates that contain MR radio frequency coils, as well as optical fibers. The medial compression plate contains optical source fibers and the lateral compression plate contains optical detector fibers, so that light transmission may be detected.
Such specialized systems are infrequently adopted as replacements for current industry standards because they are viewed as time consuming and troublesome. For example, the orientation and field gradient of the above-described integrated radio frequency coils must be recalibrated for each patient, and the system does not allow for biopsy access. A physician must therefore choose, before placing the patient into the bore of the MR instrument, whether a mass (if observed) will be optically imaged, using the specialized compression system, or biopsied, using a traditional compression system. If the physician chooses optical imaging and later, based on the optical imaging results, decides to biopsy the mass, the patient must be removed from the MR instrument, outfitted with a different compression system, placed back in the instrument and a new series of scans must be taken to relocate the mass. This extra procedure is an expensive and arduous task ties up valuable medical resources.